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This is a 3 part series blog post:
For the last couple of weeks we’ve been talking about the “digital cliff” where signals no longer work. Check out the previous installments about bandwidth, wavelength and critical distance (Part 1 and Part 2).
The critical distance is the length at which it is important to know the impedance of the cable, where the cable is now a “transmission line”. For signals like professional highdefinition video (SMPTE 292M standard), the critical distance is a little over ONE INCH. That simply means that everything is critical, cable connectors, patch panels, patch cords, adaptors…..everything in that passive line between boxes. But that still doesn’t tell you where the cliff is.
As the cable goes farther and father, there is attenuation on the cable. Bigger cable has lower attenuation than smaller cable, so if your cable is not working, you might consider changing to a bigger cable.
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This is a 3 part series blog post:
We’ve been talking about digital signals and their limitations. The first step is to ascertain the bandwidth of your signal. Our example in Tales of the Cliff – Part 1 was RS422 which has a bandwidth of 10 MHz.
Once you know the bandwidth, you can calculate the wavelength. 300 million divided by that frequency gives you the wavelength in meters. 300 million divided by 10 million equals 30 meters (about 100 feet). The critical distance is not a full wavelength but a quarter wavelength. (Think about a sine wave. Where is at its maximum effect? A quarter of a wavelength.)
So a quarter of 100 ft. is around 25 feet. And what that means is that, below 25 feet, you can use almost anything to carry RS422. After 25 feet, it is a “transmission line” and requires that you match the impedance of the source and destination devices with the impedance of the cable. For RS422, this impedance of the devices is 100 ohms, so the cable will also be 100 ohms. When you put connectors on your RS422 cable (15pin subD, most commonly) are they 100 ohms? No, they aren’t. But it doesn’t matter because they are less than 25 feet long. So you can use any connector. But that doesn’t tell you the MAXIMUM distance you can go.
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If you play with coax, short for coaxial cable, you probably know this it is available in a number of different impedances. The most common is 75 ohm, like video cable or antenna cable, but in fact our products range from 32 ohms up to 124 ohms.
Why all these different numbers? It’s not an accident of course, and there is a reason for each one. Today, we’re going to take a quick look at 50 ohm coax cable.
Belden makes hundreds of 50 ohm cables, including a whole line of ultralow loss versions (Belden 7805 to Belden 7977). The two largest versions (Belden 7976 and 7977) are shown in the photograph below. They are HUGE. The 7977 has a diameter of .600″ sixtenths of an inch! This is the largest coax cable that we make.
But first of all, why 50, or any other number? The answer can be shown in the graph below. This was produced by two researchers, Lloyd Espenscheid and Herman Affel, working for Bell Labs in 1929. Read more »
Back in the old days of analog audio, splitting a signal was no big deal. Just use a ‘Y’ adaptor. Or if you’re punching down wires, just punch another pair on top of the first. Of course, each signal will be 3 dB lower than the original, but that’s not a worry. That’s because the wavelength of audio is miles long (quarterwavelength at 20 kHz is more than 2 miles!).
While adding wires might change the impedance, it didn’t matter. You couldn’t go far enough for it to matter. But when it came to video, it was a different story. The signal is a lot higher frequency, so the wavelength is shorter. While you might get by with a BNC ‘T’, this would cause a mismatch on the two splits and could result in some reflected signals. Read more »
When you deal with high frequencies, above around 100 MHz, you have entered the zone of the “transmission line.” It has to do with the wavelength of the signal, and that is a discussion we’ll have in future blogs. But what it means is that the impedance of the cable is now important and you have to match the impedance of the source and destination devices.
This also means that everything inbetween must match the impedance chosen and, by “everything,” I mean cable, connectors, patch panels, patch cords, adaptors, bulkheads, feedthroughs – erything! It also means that any variation in impedance can affect the signal on the line. This is true for every transmission line, whether we’re talking about a 50kW RF signal going up to an antenna or an HD video signal going between boxes. Of course, in these two examples the impedance is different (50 ohms for that highpower line and 75 ohms for that video cable). Read more »
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